[1] Dalhousie Univ, Dept Phys, Halifax, NS B3H 3J5, Canada
[2] Dalhousie Univ, Dept Math & Stat, Halifax, NS B3H 3J5, Canada
[3] Univ London Queen Mary & Westfield Coll, Sch Math Sci, Astron Unit, London E1 4NS, England
[4] Univ Waterloo, Dept Appl Math, Waterloo, ON N2L 3G1, Canada